In 2009, an hour and a half lecture about sugar and obesity that I gave to the public was posted to YouTube. Given its scientific content, I wasn’t even sure if my family members would watch it. Three million views later, the video is still going strong, and my theories about sugar’s toxic effects on the body are gaining traction. I still believe that one particular form of sugar—fructose—is toxic in high dose. Yet there is still a lot of confusion about this dietary bogeyman. Here are five myths about sugar and some important distinctions about how our body processes its different forms.

Myth 1. A carbohydrate is a carbohydrate; they all have the same calories.

Half true. There are three molecules that make up all the various kinds of carbohydrate: glucose, galactose, and fructose. All three molecules have the same caloric density—4.1 kcal/gm — which is why people erroneously conclude that “a calorie is a calorie.” Glucose is what’s found in starch; it’s the energy of life; all cells in all organisms on the planet burn glucose to make energy. Galactose (the molecule exclusively found in milk sugar) is rapidly converted in the liver to glucose. Fructose (the molecule that makes sugar sweet) is also metabolized in the liver, but any excess is converted into liver fat. Chronic and excess alcohol or fructose exposure both cause fatty liver disease, which drives the pathologic process of insulin resistance, and causes the same chronic diseases — obesity, heart disease, and diabetes.

Myth 2. Fructose is turned into glucose in the body.

Maybe. If you’re energy depleted (i.e. an elite athlete), fructose can be converted to glycogen (liver starch) as a storehouse for ready energy, which can then be fished out of your liver if your body needs glucose in the future (for more exercise or if you’re starving). But most of us aren’t energy depleted, so fructose gets turned into liver fat, driving insulin resistance. This is one reason why exercise can be an antidote to excess fructose consumption.

Myth 3. People can limit their sugar consumption without any difficulty.

In fact, sugar is weakly addictive. In animal studies, fructose causes the four criteria of addiction: bingeing, withdrawal, craving, and sensitization to other addictive substances (meaning after chronic exposure to sugar, it’s easier to get hooked on another drug). In humans, fructose lights up the reward center in your brain called the nucleus accumbens on MRI; but after repeated exposure, the reward center lights up less and less, so you need more and more to achieve the same effect. Fructose has effects on the reward center similar to alcohol; and just like alcohol, can lead to a “vicious cycle” of consumption and disease.

Myth 4. High fructose corn syrup is worse than table sugar.

Studies comparing HFCS and table sugar (sucrose) head-to-head show that they are similar in terms of their metabolic effects; both are equally bad for you. HFCS is 55% fructose, sucrose is 50% fructose. However, a recent study suggests that some commercial sugar-sweetened beverages might have as much as 65% fructose, which could potentially make them worse. This has yet to be shown in large studies.

Myth 5. Fructose is natural — it’s found in fruit — so it couldn’t possibly be poisonous.

Just because a chemical is natural and safe in small quantities doesn’t mean it isn’t dangerous in large quantities; e.g. Vitamin D, iron, fluoride. Yet each of these substances are on the FDA’s GRAS (Generally Regarded as Safe) list. But they don’t have abuse potential. Fructose does. A little is OK, a lot is not. And naturally occurring fructose in fruit comes with a boatload of fiber, which limits its absorption. Refined sugar has no fiber. You absorb it all immediately — that’s how the damage is done. Of course, soft drinks are the quickest way to overload your liver; but soft drinks account for only one-third of all sugar consumed. The American Heart Association has recommended cutting our added sugar consumption back by more than two-thirds; from an average of 22 tsp/day down to 6 for women and 9 for men.

As with everything, the devil is in the details. The one thing we can agree on is that our sugar consumption has skyrocketed, from 4 teaspoons a day in 1990 to 22 teaspoons today. It needs to go back to being a once-a-week treat—something for special occasions—instead of a once-a-meal diet staple.

Dr. Robert Lustig is a professor of Pediatrics at the UCSF Benioff Children's Hospital and the author of Fat Chance: Beating the Odds Against Sugar, Processed Food, Obesity and Disease. The views expressed are solely his own.

Dr. Lustig's book is Fat Chance: Beating the Odds Against Sugar, Processed Food, Obesity and Disease.